DESCRIPTION: Almost all of the information that reaches the cerebral cortex from the outside world or from lower brain centers has to pass through the thalamus. The fibers that ascend to the thalamus, the "primary afferent fibers", provide the major information content of the messages that are passed on to the cortex. The thalamus in turn is under the influence of fiber pathways that return from the cerebral cortex to the thalamus. These have generally been regarded as having a modulatory effect on thalamocortical transmission. However, recent evidence has shown that there are two distinct types of corticothalamic fiber, one arising in layer 6 of the cortex, reasonably regarded as having such a modulatory role, but the other, from layer 5, having very different synaptic relations in the thalamus, more like the relationships established by primary afferents. These latter are more likely to provide a primary drive for thalamocortical transmission. Although the way in which cortical fibers can influence the cells of the thalamus will prove crucial for understanding higher sensory functions and motor control mechanisms, the details of which fiber types link any one region of the cortex to the thalamus are almost entirely unexplored at present. The project will identify nerve fibers that pass from several distinct areas of the visual cortex and the visual brainstem to the thalamus, revealing the functionally significant relationships established in the thalamus by cells from cortical layers 5 or 6 and the brainstem. It will show how these pathways relate to each other on individual cells and cell groups by exploiting a variety of selective fiber tracing methods in combination and studying the detailed relationships by electron microscopical serial section procedures. In this way it will be possible to categorize some of the major fiber pathways linking the visual parts of the thalamus and cortex. The outcomes of these experiments will show the extent to which single cells in the association or higher order thalamic nuclei may receive their major "drive" from more than one source, either from two different cortical areas or from cortex and an ascending pathway. If there are cells with such dual "driving" inputs, then it will become reasonable to ask about the integration that may occur between two pathways in the thalamus itself. Such a concept would shape and modify our views of the thalamus and will be a guide concerning the relevant functional properties that may be explored for any part of the association thalamus.